This guide is intended to
give a basic introduction to the subject of central heating boilers
and flues.

The installation of central heating is a
highly technical task and we strongly recommend the use of a fully accredited
professional in ALL cases. Gas central heating is installed in over
70% of UK dwellings and has an excellent safey record, but there are
inherent dangers associated with highly flammable materials and potentially
toxic fumes which demand that installations are carried out by trained
and experienced operatives.

We hope that this guide will help you to
understand the various options that are available so that you can discuss
with your chosen installer the system which is most appropriate to your
needs. Further subjects will be featured in the near future and whilst
every care has been taken in producing this section it should be noted
that the information does not constitute recommendations and is simply
a guide.

There are three traditional groups of boiler,
Free Standing, Wall Mounted & Back Boilers. There are two other
general categories, referred to as Condensing Boilers and Combination
Boilers (usually referred to as Combis). All these types are described
as follows.

Free standing
These floor-standing boilers are usually fairly slim for
fitting between kitchen units, although they may be installed in any
suitable location. Alternatively, in larger homes, free-standing boilers
may be sited in garages, out buildings or even purpose-designed boiler
rooms. These boilers are available as room sealed, fanned flue or conversional
flue versions.

Wall mounted The
vast majority of boilers these days are wall mounted. These boilers
have lighter, more compact heat exchangers, constructed from materials
such as copper, aluminium, stainless steel or lightweight cast iron.
These boilers are available as Room Sealed, or Fanned Flue versions.
The Conventional Flue version of this boiler is no longer available.

Most modern boilers of this type, are required to have a system
by-pass fitted to allow sufficient water to circulate through
the appliance to prevent kettling (the same kind of noise
associated with a singing kettle)

Back boilers (BBUs)
These boilers are fitted into the opening behind a fireplace surround
and are usually hidden behind a gas fire. A back boiler unit will save
space elsewhere in the house, but its output is limited to 16kW (55,000
BTU/hr). The noise when operating can at times be intrusive and a ventilation
path direct to outside must be maintained at all times.

If an old BBU is replaced for a new unit it will be necessary to renew
the existing flue liner as well. It is not possible to renew the fire
front only of older, existing BBUs.

Condensing boilersCondensing
boilers are not new, they have been around for many years and users,
report very significant reductions in fuel costs, in some cases. This
is possible, because this type of boiler has a much more effective 'heat
exchanger' allowing more heat to be removed from the flue products.

One of the most significant effects of this
is the 'pluming', sometimes mistaken for 'steam' coming out of the flue
terminal. This effect is brought about by water droplets held in suspension
in the flue products and although non-hazardous, it may be a nuisance
if seen constantly passing across windows. This pluming will be evident
for most of the time the boiler is operating, so care is needed in positioning
the terminal.

Although they have a somewhat higher capital
cost than non-condensing boilers, for larger dwellings (with three or
more bedrooms) they usually prove to be cost effective within several
years.

Typically, a condensing gas boiler might
have a seasonal efficiency of around 88% compared to non-condensing
boiler of about 75%, and an older type boiler at between 55% -60%. For
more information, please visit www.boilers.org.uk
website

They are as easy to install as any non-condensing
boiler, but can only be fitted to 'fully pumped' systems. During it's
operation, condensate forms and, although no more acidic than rain fall,
a permanent pipe must be run to a drain

This type of boiler is available as a wall
mounted or floor standing version.

Combination BoilersMost
wet central heating systems use a boiler to heat a copper cylinder in
the airing cupboard to provide domestic hot water. A combination boiler
(usually referred to as a 'combi') provides a means of generating hot
water for the taps from within the unit and combines this with central
heating.

By using a 'combi' neither a cylinder, nor its feed tank, are required.
Consequently, hot water running costs will be reduced very slightly
since there are no standing losses from the cylinder or boiler pipework.
Although a continuous flow of hot water is constantly available, the
flow rate is not likely to be as good as with a traditional storage
cylinder, particularly in winter.

In dwellings (often larger type) where a
number of draw-off points (taps, showers etc) are likely to be used
at the same time manufacturers advice should be sought as to the combi's
suitability. Showers fitted to this boiler must be a 'mains pressure'
type.

Because the boiler is fed directly with mains
cold water, it is vital to establish that the minimum water pressure,
specified by the manufacturer, is available otherwise the hot water
performance will suffer.

Combination boiles are by far the most popular boiler type, accounting
for over 50%of all new boiler sales within the UK. There are five types
of combi boilers:

A combi (instantaneous) boiler is the most widely used type and directly
heats incoming mains cold water to supply hot water to taps, showers
and other draw off points.

A combi (storage) boiler is a variant of the Instantaneous type and
is designed to give better hot water performance. Dependent upon the
make and model, the improvement in hot water delivery will depend upon
the size of hot water store, and this can vary considerably.

In general, the principle of operation is
that the stored water will give up it's heat to provide an improved
initial hot water delivery. Once the stored heat has been given up,
the boiler then operates as an 'instantaneous' type.

A combi (storage condensing) boiler operates
in principle as a 'storage' type but at somewhat higher efficiencies
(See Condensing Boilers).

Combined Primary
Storage Unit (CPSU) - This is a special category of storage
combi and incorporate a very large store of water (usually more than
80 litres) allowing a high hot water flow rate to taps and other draw
off points and quick heat-up to radiators.